1. Field of the Invention
The present invention relates to an optical pickup apparatus and particularly to a reading optical system of an optical pickup apparatus.
2. Background Art
The capacity of a single layer in an optical disc largely depends on the wavelength of the semiconductor laser used and the numerical aperture (NA) of the objective lens. The shorter the wavelength of the semiconductor laser, or the larger the NA, the greater the recording density can be made, with a resultant increase in the capacity of each layer. Most of the currently commercially available optical disc drives are DVD (Digital Versatile Disc) drives that employ the color of red with wavelengths in the vicinity of 650 nm and an objective lens having an NA of 0.6. Shipping has started of optical drives that exceed the recording density of DVDs, using a light source consisting of a semiconductor laser of blue-violet light with wavelengths in the vicinity of 405 nm and an objective lens with an NA of 0.85. Difficulty is expected for the development of a semiconductor laser light source having wavelengths shorter than those of such blue violet for future recording density improvement, because the wavelengths would be in the UV range. Furthermore, since the limit of NA of an objective lens in air is 1, further improvements in recording density by means of the objective lens are also becoming difficult to achieve.
Under such circumstances, use of multiple layers is suggested as a means of increasing the capacity of an individual optical disc. For example, Non-patent Document 1 discloses a ROM (Read Only Memory) having four layers. When a multilayer optical disc is irradiated with laser light, crosstalk between the layers becomes an issue because of the simultaneous irradiation of a plurality of layers. In order to address this problem, the interlayer distance may be increased. In this way, crosstalk can be reduced because laser light is focused and layers other than a target layer are displaced from the position where the laser light is focused.
However, such increase in the interlayer distance gives rise to the problem of spherical aberration. Between the recording layers, polycarbonate is used, which has a refractive index different from that of air and thus poses a cause for spherical aberration. The objective lens is designed such that its spherical aberration is minimized with respect to a particular layer. As a result, spherical aberration is caused when the focus of laser light is shifted to any of layers other than the aberration-minimized layer. Such aberration can be normally corrected by placing an expander lens system consisting of two lenses, or a liquid crystal element in front of an objective lens, and varying the distance between the two lenses or the phase of the liquid crystal element. However, it is impossible to correct large spherical aberration, given the possible range of compensation of the liquid crystal element or the need to realize a lens transfer mechanism within the small optical disc drive apparatus. Thus, it is difficult to achieve a sufficient increase in the interlayer distance in a multilayer optical disc for actual optical drive units. Consequently, some interlayer crosstalk inevitably remains in a multilayered optical disc.
In order to reduce the aforementioned crosstalk, in Patent Document 1, a minute mirror is disposed on the optical axis so as to obtain only the reflected light of interest and reduce crosstalk. This takes advantage of the fact that the position along the optical axis where the reflected light from a multilayer optical disc is focused by lenses differs between a target layer and an adjacent layer. However, because the reflected light from the optical disc is bent laterally with respect to the optical axis, the optical head inevitably needs to be increased in size. Patent Document 2 proposes a method for removing reflected light from an adjacent layer by means of a critical angle prism. In this method, which takes advantage of the fact that the reflected light from the relevant layer is converted into collimated parallel light but the reflected light from the adjacent layer becomes a diverging ray or a converging ray, rays that have assumed more than a certain angle with respect to the optical axis are removed by a critical angle prism. In this method, too, the optical head needs to be increased in size due to the use of two critical angle prisms.
Patent Document 1: JP Patent Publication (Kokai) No. 2005-302084 A
Patent Document 2: JP Patent Publication (Kokai) No. 2002-367211 A
Non-Patent Document 1: Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 778-783